Competing American astronomers who studied dozens of distant exploding stars, and in the process discovered that the universe is expanding at an ever-accelerating rate, will share the $1.5 million Nobel Prize in Physics for 2011, the Royal Swedish Academy of Sciences announced. Saul Perlmutter of Lawrence Berkeley National Laboratory in California will receive half the award; the other half will be shared by Brian P. Schmidt of Australian National University and Adam G. Riess of Johns Hopkins University and Space Telescope Science Institute in Baltimore. In 1998, cosmology was shaken at its foundations as the two research teams — one headed by Perlmutter, the other led by Schmidt and included Riess as a crucial player — presented their findings that not only is the universe expanding, but it is doing it at a faster and faster pace. Perlmutter, who is also a professor of physics at the University of California, Berkeley, heads the Supernova Cosmology Project, an international collaboration that pioneered the methods used to discover the expansion, and he has been a leader in studies to determine the nature of dark energy, the force suspected to be behind the expansion. From the start, the two teams raced to map the universe by locating the most distant supernovae. More sophisticated telescopes on the ground and in space, as well as more powerful computers and then-new CCD imagers, opened the possibility in the 1990s to add more pieces to the cosmological puzzle. The groups studied type Ia supernovae, explosions of old compact stars that are as heavy as the sun but as small as the Earth. A single such supernova can emit as much light as a whole galaxy. All in all, the two research groups found more than 50 distant supernovae whose light was weaker than expected — a sign that the expansion of the universe was accelerating. The potential pitfalls had been numerous, and the scientists found reassurance in the fact that both groups had reached the same astonishing conclusion. For almost a century, the universe has been known to be expanding as a consequence of the big bang about 14 billion years ago. However, the discovery that this expansion continues to accelerate is astounding. The acceleration is thought to be driven by dark energy, but what that dark energy is remains an enigma — perhaps the greatest currently in physics. What is known is that dark energy constitutes about three-quarters of the universe. Therefore, the findings of the two groups have helped to unveil a universe that to a large extent is unknown to science. “It’s wonderful that the Nobel Prize is being awarded for results which reflect humanity’s long quest to understand our world and how we got here,” Perlmutter said. “These are the kinds of discoveries that the whole world can feel a part of and celebrate, as humanity advances its knowledge of our universe.” Perlmutter’s Supernova Cosmology Project and Schmidt’s High-Z Supernova Search Team were jointly named the “breakthrough of the year” by Science in 1998. In 2006, the groups shared the Shaw Astronomy Prize. (See: $1M Shaw Astronomy Prize Hails Dark Energy Discovery) The 2011 Nobel Prize in Physics consists of a diploma, a gold medal and 10 million Swedish kroner (~$1.5 million). The award presentation ceremony will be held Dec. 10 in Stockholm. For more information, visit: www.nobelprize.org